1. Field of the Invention
The present invention relates to a pixel circuit and a driving method thereof, and more particularly, to a pixel circuit having an OLED (organic light-emitting diode) and a driving method thereof.
2. Description of the Related Art
Due to the potential advantages of a slim profile, wide viewing angle, fast response, high brightness, high contrast ratio, and being lightweight, OLED (organic light-emitting diode) displays promise to be an attractive display technology in the next generation. In general, a driving method for an OLED is classified into a passive matrix (i.e., PM-OLED) type and an active matrix (i.e., AM-OLED) type. The AMOLED driving method employs TFTs (thin film transistors) and storage capacitors to control the brightness and grayscale of the OLED.
The PMOLED driving method employs a simpler, cheaper circuit structure; however, the PMOLED needs high current pulses to operate to achieve the brightness that is suitable for human eyes. In addition, the brightness of the PMOLED is proportional to the current density, and thus, the operation of excessive current will degrade the lifetime and efficiency of the driving circuit.
Under the above limitations, the PMOLED is only suitable for small-sized panels such as PDAs (personal digital assistants), mobiles phones, and so on. For products with large-sized panels, the AMOLED having the properties of lower driving voltage, lower power consumption, long lifetime, faster response, and easily enhanced brightness is a better choice than the PMOLED.
The AMOLED driving method is further classified into the voltage-driving method and the current-driving method. For persons of ordinary skill in the art, the voltage-driving method suffers from the issues of mobility shift and threshold voltage shift due to variation of the manufacturing process of TFTs and the current-driving method has been developed to overcome the issues. That is, the current-driving method presents perfect compensation for the threshold voltage shift and mobility shift. However, when the size of the AMOLED panel is increasingly large, a charging problem occurs at low gray-level currents because of the large parasitic capacitive load of data lines (around 20 pF), and thus, it takes a long time to charge pixel capacitors and then the response is degraded. Therefore, it is necessary to develop a novel driving method to improve the charging ability of a conventional current-driving method.